Continuity mistake: After her bath as she gets ready for sleep Regan asks Chris, "What's wrong with me?" and her left arm goes from being under the pillow to under her head between shots. (00:43:50)
Continuity mistake: After Chris puts her to sleep and walks down the steps, Willi the maid is cleaning up Regan's mess on the living room carpet from the previous party scene. We saw a nice close-up of the wet carpet as it happened and now Willi is not even close to cleaning the right spot. In fact she's kneeling right on it, yikes. (00:44:20)
Continuity mistake: After Chris puts her to sleep and walks down the steps, Regan screams and there is a close-up of the lamp flickering on the wall. In the next shot the lighting has changed as a dark shadow appears behind the lamp. (00:44:25)
Continuity mistake: Regan screams, "Mother, make it stop" and the books near the lamp on her desk move to the right between shots. (00:44:40)
Continuity mistake: Father Dyer walks down the hall on his way to Father Karras' room and the florescent ceiling lamp closest to the window is turned off between shots. (00:44:55)
Continuity mistake: Father Dyer walks into Father Karras' room and the Chivas Regal bottle on the floor changes sides between shots. (00:45:10)
Continuity mistake: In his room, Father Dyer starts to sit next to Father Karras as he tries to console him. In the next shot he's standing in front of him. (00:45:45)
Continuity mistake: As he lays down, Father Dyer takes Father Karras' cigarette from him and puts it in his mouth. In the next shot it's back in Father Karras' right hand. (00:46:00)
Continuity mistake: As Father Karras lays down, Father Dyer shuts all the lights for him. The lamp on his desk is right over the typewriter the first time we see it than later as he turns it off it has been moved to the left. (00:46:10)
Continuity mistake: Dr. Klein talks to Chris about what Regan's test results indicate and she's touching her face with her left hand. In the next shot from the front, she's touching her face with her right hand. (00:48:05)
Continuity mistake: Dr. Klein gets socked in the mouth by Regan and blood drips out of his nose. The next time we see him, and in all the subsequent shots in this scene, there's no blood on his face. (00:53:40)
Continuity mistake: After he gets socked in the mouth, Dr. Klein gets ready to give Regan another injection and the lamp near the window and the curtain behind it move between shots. (00:54:00)
Continuity mistake: Dr. Klein and Dr. Taney arrive at the house and go up to Regan's room to see her for themselves. We see inside the room and the two doctors, Chris and Sharon are the only ones there but as they try to give her an injection, Karl appears from out of nowhere restraining her. (00:54:05)
Continuity mistake: After the second set of tests on Regan, Dr. Klein asks Chris, "Do you keep any drugs in your house?". If you look closely he faces the wrong direction when he says this as Chris has not move yet from the spot where she's standing. She moves in the next shot. (00:56:35)
Continuity mistake: As they speak about Regan, Chris sits down at his desk and a shadow appears on Dr. Klein's forehead that wasn't there in the previous shot before she walked to the seat. (00:56:55)
Continuity mistake: Chris drives home past the police cars and ambulance outside her house after Burke Dennings is killed on the steps. At first there are a bunch of people on the steps, then the next time we see it two seconds later there's a lone policeman standing there. (00:57:45)
Continuity mistake: Chris drives home past the police cars and ambulance out side her house and when she gets in there's no one there. If you look closely as she turns into her driveway, tire marks on the wet pavement can be seen in the exact same spot from a previous take. (00:57:50)
Continuity mistake: After Burke Dennings is killed, Chris drives home past the police cars and ambulance out side her house with her high beams on. In the next shot as she pulls into her driveway, her low beams are on. (00:57:55)
Continuity mistake: After Burke Dennings is killed, Chris goes upstairs and finds the window in Regan's room is wide open. It's so cold that you can see the condensation from her breath in some shots, but not in others. (00:59:10)
Continuity mistake: Chuck, the Assistant Director comes in and tells Chris and Sharon that Burke Dennings has been killed. Chris turns around and says, "Heard what" and in a bad over-dub we see her mouth move as she says something else that is been edited out. (00:59:50)
Answer: The demon wants everyone to lose hope for the girl and give up on her. Her father, having already flown the coop, just makes this goal that much easier.
TonyPH